What happened last time it was as warm as it’s going to get later this century?

Kids today will be grandparents when most climate projections end—does the past have more hints? Ars Technica HOWARD LEE – 6/18/2018

“What’s past is prologue”- Shakespeare’s The Tempest.

The year 2100 stands like a line of checkered flags at the climate change finish line, as if all our goals expire then. But like the warning etched on a car mirror: it’s closer than it appears. Kids born today will be grandparents when most climate projections end.

Why is there a yawning gap between end-century projections and what happened in Earth’s past? Are past climates telling us we’re missing something?

Time

One big reason for the gap is simple: time.

Earth takes time to respond to changes in greenhouse gases. Some changes happen within years, while others take generations to reach a new equilibrium. Ice sheets melting, permafrost thawing, deep ocean warming, peat formation, and reorganizations of vegetation take centuries to millennia.

These slow responses are typically not included in climate models. That’s partly because of the computing time they would take to calculate, partly because we’re naturally focused on what we can expect over the next few decades, and partly because those processes are uncertain. And even though climate models have been successful at predicting climate change observed so far, uncertainties remain for even some fast responses, like clouds or the amplification of warming at the poles.